1. Field of the Invention
The present invention relates generally to optical fiber, and more particularly to a polarization maintaining and/or single polarization optical fiber.
2. Technical Background
Polarization maintaining (PM) and single polarization (SP) optical fibers are useful for ultra-high speed transmission systems and many other applications. One type of prior polarization maintaining fiber includes, as shown in FIG. 1, a central core 10 surrounded by a cladding 11. Core 10 and cladding 11 are formed of conventional materials employed in the formation of optical waveguides. The refractive index of the core material is greater than that of the cladding material. By way of example only, core 10 may consist of silica containing one or more dopants which increase the refractive index thereof, such as germania. Cladding 11 may comprise pure silica, silica containing a lesser amount of dopant than core 10, or silica containing one or more down dopants, at least one of which is an oxide of an element such as boron or fluorine which lowers the refractive index of silica. In FIG. 1, diametrically opposed relative to core 10, are two stress rods 12 formed of silica doped only with B2O3 concentration being 20 to 25 wt %. These Boron (B) doped regions have a coefficient of thermal expansion (CTE) different from that of cladding material 11. When such a fiber is drawn, the longitudinally-extending stress rods 12 and the cladding regions will shrink different amounts, whereby stress rods 12 will be put into a state of tension or compression depending upon the CTE thereof relative to that of the cladding 11. A strain induced birefringence, which is thus induced by anisotropic thermal stress induced by the mismatch of CTEs between stress rods 12 and the surrounding regions 11, reduces coupling between the two polarized fundamental modes (with orthogonal polarization directions). However, B doped stress rods contribute to fiber attenuation. In order to avoid high fiber attenuation, it is known that the stress rods should be placed relatively large distance away from the fiber core.
Another approach is to manufacture the stress rods from either GeO2, P2O5, or Al2O3 because they introduce relatively large amount of stress into the fiber. However, although there materials contribute to significant amount of stress effect, and thus birefringence, they increase refractive indices of the rods, raising the refractive index of the stress rods higher than the fiber cladding which is typically formed of pure silica. A higher index stress rod 12 can act as an additional waveguiding core, which is not desirable.
In order to take the advantage of the higher stress induced birefringence by GeO2, P2O5, or Al2O3 while avoiding the unwanted wave-guiding effect, additional index lowering dopant (i.e., Boron or Fluorine) is introduced into the stress rods. However, it was believed that, in order to keep the fiber loss within acceptable range, such stress rods (or stress members) have to a relatively large distance b away from the core, which necessitated high amount of dopants to produce sufficient birefringence.